Take the silencer off, sounds like it could be blocked. Try and run it without the silencer, of course leave the expansion chamber on.
After watching your video this is my initial thought, a partially blocked silencer will pass enough exhaust gas at lower RPM but not as the revs get higher.
Let us know how you get on, if it's not the silencer then we'll suggest other things to check.
All the best, Dave.
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